1. Field
The present invention relates generally to data communication, and more specifically to techniques for performing iterative detection and decoding for a MIMO-OFDM communication system.
2. Background
A multiple-input multiple-output (MIMO) communication system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, with NS≦min {NT, NR}. Each of the NS independent channels is also referred to as a spatial subchannel of the MIMO channel and corresponds to a dimension. The MIMO system can provide improved performance (e.g., increased transmission capacity) over that of a single-input single-output (SISO) communication system if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
A wideband MIMO system typically experiences frequency selective fading, i.e., different amounts of attenuation across the system bandwidth. This frequency selective fading causes inter-symbol interference (ISI), which is a phenomenon whereby each symbol in a received signal acts as distortion to subsequent symbols in the received signal. This distortion degrades performance by impacting the ability to correctly detect the received symbols. As such, ISI is a non-negligible noise component that may have a large impact on the overall signal-to-noise-and-interference ratio (SNR) for systems designed to operate at high SNR levels, such as MIMO systems. In such systems, equalization may be used at the receivers to combat ISI. However, the computational complexity required to perform equalization is typically significant or prohibitive for most applications.
Orthogonal frequency division multiplexing (OFDM) may be used to combat ISI, and achieves this without the use of computationally intensive equalization. An OFDM system effectively partitions the system bandwidth into a number of (NF) frequency subchannels, which may be referred to as sub-bands or frequency bins. Each frequency subchannel is associated with a respective subcarrier upon which data may be modulated. The frequency subchannels of the OFDM system may experience frequency selective fading (i.e., different amounts of attenuation for different frequency subchannels), depending on the characteristics (e.g., multipath profile) of the propagation path between the transmit and receive antennas. With OFDM, the ISI due to the frequency selective fading may be combated by repeating a portion of each OFDM symbol (i.e., appending a cyclic prefix to each OFDM symbol), as is known in the art.
A MIMO system may thus advantageously employ OFDM to combat ISI. The frequency subchannels of the MIMO-OFDM system may experience different channel conditions (e.g., different fading and multipath effects) and may achieve different SNRs. Moreover, the channel conditions may vary over time. Consequently, the supported data rates may vary from frequency subchannel to frequency subchannel and from spatial subchannel to spatial subchannel, and may further vary with time. To achieve high performance, it is necessary to properly code and modulate the data at the transmitter (e.g., based on the determined channel conditions) and to properly detect and decode the received signals at the receiver.
There is therefore a need in the art for techniques to detect and decode signals that may have been (flexibly) coded and modulated based on one or more coding and modulation schemes, e.g., as determined by the channel conditions.